breakup

How do you completely cut someone off that meant so much to you. My bf and I broke up but I still keep in contact with him because he blows my phone up and I give in. It's like I don't know how to let go we've been together since I was 17 I'm now 22. I have a scary feeling that he's going to be with the girl he cheated on me with and it drives me crazy and makes my heart break maybe that's why I give him. How do you stop caring about a person that treats you like shit. How do you just wake up and decide FUCK HIM I'm moving on??? I need help I need answers or I'm going to lose it!!